Combi, system or regular — choosing the right boiler type

The three types in one paragraph each

• Combi: heats water on demand, straight from the mains. No cylinder, no tanks, no airing cupboard. Excellent for smaller homes with one bathroom and good mains pressure.
• System: heats a pressurised hot water cylinder. Expansion vessel and pump are inside the boiler; no loft tanks. Good for homes with two or more bathrooms, or where several outlets run together.
• Regular (heat only): the traditional arrangement with a cylinder plus cold water storage tanks in the loft. Usually retained rather than chosen new, but it suits properties with poor mains pressure or gravity-fed legacy pipework.

Side-by-side comparison

Mains pressure is the deciding factor for a combi

A combi can only deliver what the mains gives it. Flow rate matters more than pressure: a combi needs a healthy litres-per-minute figure at the stopcock to produce a shower anyone enjoys. Plenty of rural properties on long private supplies, shared spurs or old lead-replacement runs simply do not have it.

Any installer proposing a combi should measure your incoming flow rate and static pressure before quoting. If they have not, the quote is a guess. Where flow is marginal, a system boiler with an unvented cylinder, or an accumulator, gives a far better result than forcing a combi onto a weak supply. Our guide to low mains water pressure explains what can and cannot be improved.

Bathrooms and household size

The rough rule: one bathroom and up to about three occupants, a correctly sized combi is usually the best answer — simplest, cheapest, no stored water, no cylinder losses. Two bathrooms used at the same time, a bath that needs filling quickly, or a household of four or more, and a system boiler with a properly sized unvented cylinder wins comfortably.

It is worth being honest about usage patterns rather than theoretical capacity. A family of five where everyone showers between seven and eight in the morning has a very different demand profile from a couple who bathe in the evening, even though the house is the same size.

Reasons to keep a cylinder rather than convert

• You have, or may add, solar thermal or an air source heat pump — both need stored hot water, and ripping out a cylinder now means paying to reinstate it later.
• Mains flow rate is below what a combi needs for a decent shower.
• The property has two or more bathrooms in genuine simultaneous use.
• You want hot water to survive a boiler fault — a cylinder with an immersion heater gives you a backup that a combi cannot.
• The airing cupboard is genuinely useful in a damp climate, and removing the cylinder removes the drying space with it.

Reasons to convert to a combi

• You are short of space and would rather have the cupboard back.
• Single bathroom, good mains flow, and no plan to add heat pumps or solar.
• You are tired of running out of hot water because the cylinder is undersized or badly insulated.
• You want to eliminate loft tanks entirely — sensible in properties where freezing pipes in the loft have been a recurring problem.

One decision that outlives the boiler

Whatever you choose, the pipework and cylinder decisions last far longer than the appliance. A boiler is a fifteen-year purchase; the system layout is a thirty-year one. If there is any realistic chance of a heat pump in the next decade, keeping or installing a well-insulated cylinder and generously sized radiators makes that transition straightforward instead of expensive.

We survey, measure the mains flow, count the real hot water demand and then recommend. If a straightforward combi swap is right for your house, that is what we will quote — it is the cheaper job, and we would rather be right than upsell.

Frequently asked questions

Can I run two showers off a combi?

Generally not comfortably. A combi splits its flow between outlets, so two simultaneous showers each get roughly half. For genuine simultaneous use, a system boiler with a correctly sized unvented cylinder is the right answer.

Is a combi cheaper to run?

It avoids cylinder standing losses, which helps. But a cylinder in a well-insulated modern jacket loses very little, and a system boiler can run at lower flow temperatures more of the time. The difference is smaller than people expect.

What flow rate does a combi need?

As a working guide, you want a strong mains flow at the kitchen tap for a satisfying shower — well above the bare minimum an appliance will technically operate on. We measure it on survey rather than estimating.

Do I need a system boiler for a heat pump later?

A heat pump replaces the boiler entirely, but it does need a hot water cylinder. Keeping a cylinder and sizing radiators generously now makes a later heat pump installation far simpler and cheaper.